1. Field of the Invention
The present invention relates to magnetic resonance imaging, and in particular to the application of this technique to acquiring information about flowing liquids, in particular flowing blood in a live patient.
This invention can be used in conjunction with one, some or all of the inventions described in the following patent applications all assigned to the same assignee as the present application and filed Jan. 6, 1997: (1) U.S. Ser. No. 08/779,020 and entitled Magnetic Field Measurement; (2) U.S. Ser. No. 08/779,021 and entitled Radio Frequency Coil Switching; (3) U.S. Ser. No. 08/779,016 and entitled Phase-Error Correction in Magnetic Resonance Machines, now abandoned; and (4) U.S. Ser. No. 08/779,018 and entitled Gradient Coils in Magnetic Resonance Imaging Machines.
2. The Prior Art
The principles behind magnetic resonance imaging have been described in many publications, for instance "The Principles of Magnetic Resonance" by D. B. Lougmore, British Medical Bulletin (1989), Vol.45, No.4, pages 848 to 880. In general terms, the imaging is conducted by the excitation of hydrogen nuclei in a subject to be studied, the subject being positioned in a magnetic field and the excitation being carried out by a radio frequency energy field. The hydrogen nuclei resonate at frequencies according to the strength of the magnetic field in which they are located, and from these resonances it is possible to build up a three dimensional picture of the subject under consideration.
More particularly, a fixed very strong magnetic field is provided, upon which are super-imposed gradient magnetic fields generated by electromagnets known as gradient coils, and by proper control of these coils it is possible to control the magnetic fields at points across the image volume. As is known, these gradient fields can be controlled to change very fast.
The "raw data" obtained from the imaging machine is data concerning the frequencies of the resonances referred to above, and, mathematically, this data is considered to be in the frequency domain, or k-space. Again, as is well known such data can be transformed using Fourier transforms to give the spatial representation of the data which is the final image.
More recently, a number of methods have been developed for quantitatively studying flow, for instance blood flow in a heart during the heart cycle, by use of magnetic resonance imaging. In order to obtain a complete picture of such blood flow it is necessary to obtain information in seven dimensions, that is 3 spatial, 3 velocity and 1 time dimension. Obtaining such high quantities of information can mean that the overall imaging process is impracticably long.
In general therefore, time limitations have meant that various compromises have had to be made when such flow information is required. One option which has been pursued is to remove at least one dimension of information in order to study the other dimensions fully.
In an article, "Rapid 7-Dimensional Imaging of Pulsatile Flow" by D. N. Firmin et al published in the Proceedings of the Symposium on Computers in Cardiology, London, IEEE, September 1993, a particular method of 7-d imaging has been described. In this method the data acquisition time was reduced by restricting the field of interest to a particular rectangular cross section column of the imaging space. It is possible to do this without loss of important flow information because blood flow regions of interest are normally relatively small. Even if the flow patterns within the heart chambers are to be studied, the maximum spatial dimension of interest is usually only a few centimetres.
In the above mentioned method known techniques are used to selectively excite the selected column and different known types of imaging are used to cover the total image volume. In particular a frequency encoding gradient was used to move along the long axis of the column, while a limited number of phase encoding steps are used across the short axes of the column. Phase velocity mapping is used to quantify velocities in the 3 dimensions. These various phase techniques are well known in the art of magnetic resonance imaging, and further details of these can be found in the above mentioned article and the articles referred to therein.
While the method referred to above was a considerable improvement on the previous techniques it could still take up to 30 minutes to acquire a complete set of the required data, and it will be appreciated that since such studies are carried out on live patients, further reduction of the data acquisition time is desirable and advantageous.